How can i slow down the Animation Speed of ALL Animations in a Project?
It depends on if you want to slow down the animations or the render:


To slow down the animations themselves, go into the Dopesheet, put your blue cursor at frame 0 (the cursor position will determine the operation center), select all and scale up with S + drag.





To slow down the render itself, i.e. increase the amount of frames as if you were shooting in slow motion, go into the Properties panel > Output > Dimensions and play with the Time Remapping values. The Old and New values allows you to set the ratio between the current speed and the one you want, for example to double the time you need to choose a ratio of 1(Old)/2(New), to add 20% choose 10/12 or 5/6, etc. If you’ve slowed down by a half you’ll need to double the Frame End value, otherwise you’ll only render half of the whole animation.